if (code == null || listener == null) {
      return new MockResponse()
          .setResponseCode(404)
          .setBody("unexpected request");
    }

    try {
      OAuthSession session = slackApi.exchangeCode(code, redirectUrl());
      listener.sessionGranted(session);
    } catch (IOException e) {
      return new MockResponse()
          .setResponseCode(400)
          .setBody("code exchange failed: " + e.getMessage());

    if (team != null) {
      builder.addQueryParameter("team", team);
    }

    return builder.build();
  }

  /** See https://api.slack.com/methods/oauth.access. */
  public OAuthSession exchangeCode(String code, HttpUrl redirectUrl) throws IOException {
    HttpUrl url = baseUrl.newBuilder("oauth.access")
        .addQueryParameter("client_id", clientId)
        .addQueryParameter("client_secret", clientSecret)

import okhttp3.Response
import okhttp3.Route
import okhttp3.internal.connection.RealCall
import okio.Sink
import okio.Socket
import okio.Source

/** Encodes HTTP requests and decodes HTTP responses. */
interface ExchangeCodec {
  /** The connection or CONNECT tunnel that owns this codec. */
  val carrier: Carrier

  /** Returns true if the response body and (possibly empty) trailers have been received. */
  val isResponseComplete: Boolean

import okhttp3.Headers
import okhttp3.OkHttpClient
import okhttp3.Protocol
import okhttp3.Request
import okhttp3.Response
import okhttp3.internal.headersContentLength
import okhttp3.internal.http.ExchangeCodec
import okhttp3.internal.http.ExchangeCodec.Carrier
import okhttp3.internal.http.HTTP_CONTINUE
import okhttp3.internal.http.RealInterceptorChain
import okhttp3.internal.http.RequestLine
import okhttp3.internal.http.StatusLine

import okhttp3.ResponseBody
import okhttp3.internal.http.ExchangeCodec
import okhttp3.internal.http.RealResponseBody
import okio.Buffer
import okio.ForwardingSink
import okio.ForwardingSource
import okio.Sink
import okio.Socket
import okio.Source
import okio.buffer

/**
 * Transmits a single HTTP request and a response pair. This layers connection management and events
 * on [ExchangeCodec], which handles the actual I/O.
 */